Generally, in an automatic transmission, shifting to a shift speed is commanded according to a vehicle speed and a throttle opening, and, to achieve the commanded shift speed, a transmission path of a speed change mechanism is changed by a clutch-to-clutch shift.
As a hydraulic control unit for the above-mentioned clutch-to-clutch shift, one example is disclosed in Japanese Patent No. 3331844. FIG. 7 shows a change of hydraulic pressure during an upshift by the hydraulic control unit. In FIG. 7, PA is a hydraulic pressure command value for a (first) friction engagement element on an engaging side (hereinafter called “engaging-side hydraulic pressure”), and PB is a hydraulic pressure command value for a (second) friction engagement element on a releasing side (hereinafter called “releasing-side hydraulic pressure”), and both of them are output from a control unit to a linear solenoid valve, which is a pressure regulating means. Note that the symbols are the same as those used in FIG. 3A describing an exemplary embodiment of the present invention, and the details are described later with reference to FIG. 3A.
After a servo start control by which a piston is stroked to close a backlash of the first friction engagement element, the engaging-side hydraulic pressure PA is increased depending on an input torque, by a first sweep control in which the pressure sweeps with a predetermined gradient toward a target hydraulic pressure PTA immediately before an inertia phase starts, and by a second sweep control in which the pressure sweeps from the target hydraulic pressure with a gradient δPTA lower than the predetermined gradient (torque phase control). When an input rotational speed is changed by the pressure increase, an inertia control starts, and the pressure sweeps up while being controlled by feedback of the amount of change in the input rotational speed and increases with a high gradient near the completion of the shift (completion control), and thus the gear shift is completed.
On the other hand, the releasing-side hydraulic pressure PB is calculated by a function of the engaging-side hydraulic pressure PA and the input torque, and is swept down depending on the increase of the engaging-side hydraulic pressure PA. Therefore, after the servo start control, the pressure sweeps down with a predetermined gradient corresponding to the first sweep control toward the target hydraulic pressure PTA for the engaging-side hydraulic pressure PA, and, corresponding to the second sweep control after the target hydraulic pressure PTA, sweeps down with a gradient δPC lower than the above-mentioned predetermined gradient. When the inertia control starts, the pressure sweeps down with a comparatively high gradient δPE1, and the releasing-side hydraulic pressure PB becomes zero.